Data storage systems are arrangements of hardware and software that typically include multiple storage processors coupled to arrays of non-volatile data storage devices, such as magnetic disk drives, electronic flash drives, and/or optical drives. The storage processors service host I/O operations received from host machines. The received I/O operations specify storage objects (e.g. logical disks or “LUNs”) that are to be written, read, created, or deleted. The storage processors run software that manages incoming I/O operations and that performs various data processing tasks to organize and secure the host data received from the host machines and stored on the non-volatile data storage devices
Many conventional data storage systems use RAID (redundant array of independent disks) technology to store large volumes of data across numerous physical storage drives without requiring user machines to address each individual disk. RAID systems also frequently store data redundantly so data can be recovered if one or more storage drives fail unexpectedly.
Some data storage systems using RAID technology include spare storage drives which are reserved as “hot spares” in case one or more storage drives fail. In the event of a drive failure, the system can be “rebuilt” by reconstructing data from the failed drive using data from the remaining drives and then writing the recovered data to a spare drive. Once all the recovered data has been written to the spare drive, the system can continue normal operation.